Well I don't necessarily have a favorite. But I do enjoy the following books:
The Science of Hitting - Ted Williams. I think that everyone should get this book, whether you are a pitcher or a hitter, or both. This will help you regardless.
The Hitting Edge - Robinson. I guess you can tell that I like hitting books. Haha.
The Mental ABC's of Pitching - ?. Haha. Fooled you! But this is a great book. I recommended it to any pitcher.